At its peak in the 11th to 13th centuries A.D., the medieval settlement of Angkor in Cambodia was home to more than a million people.

Sprawling north of Cambodia's Tonle Sap Lake, the largest fresh water lake in Asia, greater Angkor was a "hydraulic city," linked by an elaborate water management network that covered more than 1,000 square kilometers (about 400 square miles). It provided a stable water supply despite the unpredictable monsoon season.